The present invention relates to a motor with a lead screw and, more particularly, to a motor with a lead screw which can be short in the axial direction when it is employed to drive, e.g., a recording or reproducing head to track a disk-shaped recording medium and which has good durability.
Conventionally, in one magnetic disk apparatus, among various magnetic disk apparatuses and the like, a lead screw serving as a feed screw is formed on the output shaft of a stepping motor, a desired head is mounted on a carriage which engages with the lead screw, and the stepping motor is driven in a predetermined manner based on a command signal, thereby causing the head to access a desired track of a disk. In this apparatus, when there is conversion of a rotational motion into a rectilinear motion, it is indispensable that the amount of rotation of the stepping motor be accurately converted into a translation of the carriage without error by eliminating any backlash of the lead screw in the axial direction.
In a normal operation state in which an excessive load is not applied, a pre-load mechanism which prevents the lead screw from moving in the axial direction is conventionally provided. More specifically, in an often employed arrangement, one end of the rotating shaft of the motor is fixed in position and axially pivotally supported by a pivot bearing, and the other end of the rotating shaft of the motor is biased in the axial direction by a leaf spring having a spring arm of a predetermined length supported in a substantially cantilevered manner.
The pre-load mechanism of this arrangement employing the leaf spring is advantageous in that its structure can be simplified. However, in order to obtain a desired biasing force, the spring arm of the leaf spring is caused to flex in the longitudinal direction of the lead screw in a cantilevered manner. Hence, the variations in the entire length of the lead screw in the axial direction adversely influence the biasing force of the pre-load mechanism. As a result, a sufficient pre-load precision cannot be guaranteed.
A spring arm may come into contact with the shaft end of the lead screw in a tilted state depending on the variations in entire length of the lead screw in the axial direction. When the spring arm contacts the shaft end in a tilted state in this manner, for example, a biasing force undesirable for the pre-load mechanism is generated, e.g., a component of force is generated in the radial direction, leading to non-uniform wear of the pre-load mechanism. In particular, when the leaf spring is made small, a sufficiently long spring arm cannot be obtained, making it further difficult to prevent the spring arm from coming into contact with the shaft end in a tilted state, as described above.
For this reason, Japanese Patent Laid-Open No. 6-133490 proposes a motor with a lead screw, which has a biasing member, in place of a leaf spring, that can translate as it is formed with a plurality of spiral slits.
The arrangement of this prior art will be briefly described with reference to the accompanying drawings. FIG. 10A is a sectional view of the main part of the arrangement of this prior art. FIG. 10B is a front view of the biasing member. Referring to FIG. 10A, a lead screw 105 is pivotally supported by a bearing 103 of a base 101. One end portion of the lead screw 105 is axially supported by a pivot bearing (not shown). The end portion of the lead screw 105 shown in FIG. 10A is biased by a biasing force which is generated upon deformation of a biasing member 111, so that the lead screw 105 is moved in the axial direction, thereby eliminating any backlash in the axial direction. A permanent magnet 107 made of a predetermined magnetic material and rotatable within a coil 108 fixed to the base 101 through a mount plate 106 is fixed to the lead screw 105. A rotating magnetic field is generated by the permanent magnet 107 and the coil 108, thereby rotating the lead screw 105 for a predetermined amount.
A plurality of spiral slits 111b are formed in the biasing member 111, as shown in FIG. 10B, to enable an acting portion 111a of the biasing member 111 to translate, so that a biasing force acts on the end portion of the lead screw 105. Also, as shown in FIG. 10A, a lid member 112 is provided to prevent the biasing member 111 from deforming more than its maximum deformation amount.
With the above arrangement, since the spring arm of the leaf spring can be prevented from coming into contact with the shaft end of the lead screw in a tilted state, a component of force is not generated in the radial direction. Since generation of an undesirable biasing force is eliminated, non-uniform wear and the like can be prevented.
A motor with a lead screw according to the conventional arrangement, which is widely put into practical use and which has a pre-load mechanism employing a leaf spring, will be described with reference to the schematic diagram of FIG. 11. Referring to FIG. 11, a lead screw 105 is pivotally supported by a bearing 12 of a base 101, and its end portion is axially supported by a pivot bearing 202. A leaf spring 109 deforms with respect to the right end portion of the lead screw 105, thereby eliminating any backlash of the lead screw 105 in the axial direction. The leaf spring 109 is fixed to a lid member fixed to the end portion of a coil 108 serving as a stator. A permanent magnet 107 made of a predetermined magnetic material and rotatable within the coil 108 fixed to the base 101 is fixed to the lead screw 105. When a rotating magnetic field is generated by the permanent magnet 107 and the coil 108, the lead screw 105 is rotated for a predetermined amount. A biasing mechanism which has the above arrangement and which performs biasing in the axial direction with deformation of the leaf spring 109 is widely put into practical use as it can be fabricated easily. However, in order to obtain a desired biasing force, the leaf spring is caused to flex in the longitudinal direction of the lead screw in a cantilevered manner, as shown in FIG. 11. Hence, variations in the entire length of the lead screw in the axial direction adversely, directly influence the biasing force of the pre-load mechanism. As a result, a sufficient pre-load precision cannot be guaranteed.
The leaf spring 109 may come into contact with the shaft end of the lead screw in a tilted state depending on the variations in the entire length of the lead screw in the axial direction. When the leaf spring 109 contacts the shaft end in a tilted state in this manner, a biasing force F, undesirable for the pre-load mechanism, is generated, e.g., a component of force in the radial direction is generated, leading to non-uniform wear of the pre-load mechanism. In particular, when the leaf spring is made small, a sufficiently long spring arm cannot be obtained, making it more difficult to prevent the spring arm from coming into contact with the shaft end in a tilted state, as described above.
For this reason, a motor with a lead screw as proposed, as in Japanese Utility Model Laid-Open No. 6-21383, has a complicated pre-load mechanism that applies a pre-load to a pivot bearing with a leaf spring. Also, in an arrangement shown in, e.g., Japanese Utility Model Laid-Open Nos. 60-2367 and 63-77471, the two ends of a lead screw 105 are axially supported by pivot bearings so that the lead screw 105 can rotate without wobbling. Japanese Patent Laid-Open No. 6-105525 discloses an arrangement in which the shaft end of a lead screw is axially supported by a bearing constituted by a metal bearing and a leaf spring. In any of these arrangements, however, the size of the motor is undesirably increased in the axial direction.
According to the arrangement disclosed in Japanese Patent Laid-Open No. 6-133490 described above, a pressing portion having spiral slits is provided in order to apply a pre-load to the end portion of the lead screw with a biasing member which can translate. Since at least the spring arm of the leaf spring can be prevented from coming into contact with the shaft end of the lead screw in a tilted state, a component of force is not generated in the radial direction. Since generation of an undesirable biasing force is eliminated, non-uniform wear and the like can be prevented. However, as the lead screw is increased in length in the axial direction, the motor cannot be made small.